1. Field of the Invention
The present invention relates to an aperture apparatus for camera zoom lenses and particularly to a zoom lens aperture device having an open aperture diameter variable in operative association with zooming.
2. Description of the Prior Art
Many conventional zoom lenses include an aperture having its diameter variable in co-operation with zooming or focusing to compensate the change of effective beam diameter at various aperture positions such that F-value will be maintained constant.
U.S. Pat. No. 4,264,178, which was issued on Apr. 28, 1981, discloses one of this type of zoom lens in which an aperture being closed from its open aperture diameter may be controlled such that the aperture will be positioned exactly at a preset aperture value in any selected zooming position.
Such an aperture apparatus comprises first and second co-operating members located between a connection of the aperture with the camera and an aperture blade assembly, the first and second members being connected with each other in a transmitting portion, which is provided with a predetermined gap .delta.. The aperture apparatus further includes a first spring for biasing the first member in the direction in which the aperture blade is closed, and a second spring for biasing the second member in the direction in which the aperture blade is opened.
The movement of the first co-operative member under the influence of the first spring will thus be transmitted to the second co-operating member after the gap .delta. in the transmitting portion has been closed, so that the second member can be moved against the action of the second spring. The movement of the first member is stopped when the aperture being closed reaches a preset aperture value.
During the closing of the aperture, the second member and aperture blades have inertia due to which after the first co-operating member has already been stopped, the second co-operating member tends to be further moved in such a direction that the gap .delta. is again formed in the transmitting portion. Thus, the aperture blades will vibrate at the final closing step. Such a vibration retards the complete stoppage of the aperture blade assembly. Particularly, when the aperture diameter is to be controlled to smaller values, the above vibration will adversely affect the exposure condition.